The present invention relates to the cutting of steel billets into selected lengths from a continuous cast strand or the like and, more particularly, to an apparatus and method for eliminating the formation of slag on cuts made with an oxy-fuel torch. Oxy-fuel cutting torch systems are well known and commonly used to cut billets and other steel sections into selected lengths. For example, in a continuous casting operation, a continuous cast strand is cut into billets lengths as the strand exits the caster. A cutting torch is mounted to move with the cast steel strand and to make a lateral separating cut through the strand perpendicular to its longitudinal axis and direction of movement. In one conventional cutting torch mounting assembly, the torch is attached to a carriage which may be clamped directly to the billet to move therewith such that the cutting torch and the cutting jet depending downwardly from the torch may be moved across and cut the billet while the continuous cast strand moves longitudinally from the casting machine.
As the oxy-fuel cutting jet cuts through the billet, a slit or kerf is formed and molten steel and iron oxide flows downwardly between the faces of the metal defining the kerf under the influence of gravity and the force of the cutting jet. Much of the molten material drops below the billet for collection and disposal, but some of the molten material adheres to and forms a slag bead along the lower edges of the kerf. If the slag accumulation is not immediately removed or prevented from forming, it will eventually harden and have to be removed in a secondary operation. The slag must be removed otherwise it will adversely affect subsequent forming operations performed on the billet and may result in defects in the final steel product. Secondary operations, such as scarfing, are tedious, time consuming and costly and, in addition, the removal of slag once it has substantially hardened may also result in the removal of base material. Slag formation may be minimized with slow speed cutting, but slow cutting speeds are incompatible with the more rapid and efficient continuous casting processes requiring faster billet cutting speeds.
Thus, there has been a need and desire in the industry for a method and apparatus which will eliminate or preclude the formation of slag on the lower edges of the kerf in billets and similar shapes cut at higher speeds with an oxy-fuel cutting torch. The method and apparatus must be simple and effective, yet operate in the extremely hostile environment typically encountered.
The prior art is repleat with attempts to prevent slag formation on the lower edges of the kerf formed in a flame cut billet or other shape. In general, it has been discovered and is well known that directing a fluid stream at the molten slag as it forms on the edges of the kerf will tend to blow it away and thereby preclude its adherence. The fluid stream may comprise air, oxygen, water, mixtures thereof, or other gases or liquids. In U.S. Pat. No. 2,289,786, a narrow stream or jet of oxygen is directed at the lower edge of the leading face of the kerf formed by a gas cutting nozzle. The nozzle providing the oxygen stream is supported on a mechanism which causes the nozzle to closely follow the advancing face of the kerf as the billet is cut and to direct the stream of oxygen at the point of emergence of the slag from the kerf. However, the moving nozzle support apparatus is in direct contact with the hot billet and the oxygen nozzle is disposed directly beneath the cutting torch jet as the nozzle traverses the underside of the billet. Although the apparatus is described as being capable of oxidizing and blowing the slag away from the base metal, it is also known that the extremely hostile environment makes the apparatus totally impractical from the standpoint of effective and useful life.
U.S. Pat. No. 2,266,552 shows the use of an oxygen nozzle or combination of nozzles for directing a low velocity oxygen stream at the slag bead in a secondary operation after the billet has been cut by hot-sawing. The low velocity oxygen stream is intended to completely oxidize and burn off the slag from the sawed body. As disclosed, the oxygen streams are disposed closely adjacent the slag beads and, in one embodiment, to travel with, but disposed against the teeth of, the advancing saw blade. The narrowly concentrated low velocity stream requires the nozzle to be positioned closely adjacent the slag bead and, as indicated, is either provided as a secondary operation or must be disposed directly in the hostile and potentially damaging environment when performed concurrently with the sawing operation.
U.S. Pat. No. 4,336,078 shows a slag removal oxygen nozzle which is adapted to be mounted for travel with the gas cutting nozzle as the latter travels above and cuts through an ingot, slab or the like. As in the patents previously described, the slag removing oxygen jets provide narrow, small diameter streams of oxygen which are oriented to impinge directly on the lower edge of the kerf immediately adjacent the advancing edge thereof as the cut progresses. In particular, the oxygen jet supporting apparatus is adapted to move within and through the kerf as it is formed. Again, as with the previous art described, the effectiveness of the slag removal provided by this apparatus requires its orientation and movement within the extremely hostile environment where it is not likely to last for any practically adequate period of time. In addition, advances in cutting torch technology have provided the ability to make cuts with much narrower kerfs, and the carrying apparatus disclosed in this patent is much too wide to practically fit within narrow kerfs formed with today's technology.
U.S. Pat. No. 4,405,382 describes a method and apparatus for removing a slag bead in a secondary operation. A stream of oxygen is directed against the cut surface (i.e. perpendicular to the direction of the cut) immediately above the lower edge of the kerf under which slag bead forms. The narrow and highly directed oxygen jet is disposed closely adjacent the surface of the cut and provides a so-called "scarfing" action which usually results in the removal of base material as well.
Published German patent application (Offenlegungsschrift) 29 25 419, dated Jan. 8, 1981, shows a similar apparatus in which a pair of oxygen nozzles are attached to a carriage to move under a slab being cut with an oxy-fuel torch and with the oxygen nozzles disposed closely adjacent the lower edge of the advancing cut face of the kerf. Similarly, European Pat. No. 0 121 589 dated Oct. 25, 1987, shows a lower oxygen nozzle attached to a cutting torch for movement therewith as the torch moves over the top of and cuts the billet. The oxygen nozzle is attached to a laterally curved supply line to avoid contact with the billet. However, the outlet from the oxygen nozzle is still disposed directly beneath the cutting torch in alignment with the cutting jet during at least a portion of the cutting cycle where it is likely to be damaged or destroyed. In addition, the system requires a separate mechanism to provide oxygen nozzle movement. One embodiment shows a transverse rectangular slot at the outlet opening of, the nozzle which has a wide horizontal dimension and a narrower vertical dimension. However, the nozzle is nevertheless mounted to be disposed close to the billet and to move with the cutting torch during the cutting operation.
Published European Patent application 0 235 588, dated Sept. 9, 1987, discloses an apparatus and method in which an auxiliary horizontally disposed cutting torch is used to make a secondary scarfing cut along the lower edge of the kerf after the billet has been cut with a conventional vertically disposed cutting torch.
Thus, although the prior art recognizes the ability to oxidize and blow-away slag as it tends to form on the lower edges of the kerf in torch-cut steel shapes concurrently with the cutting process, the prior art is characterized by the use of narrow, highly directed oxygen or other gas jets which are disposed directly adjacent the advancing edge of the kerf.